Particulate minerals such as kaolins, talcs, calcium carbonate, calcium sulfate and various micas have long been utilized as inert extenders or fillers in polymers or similar matricies. Aside from providing economic advantages in extending the more costly polymeric material, such fillers serve in many instances to improve the properties of the resultant plastics with respect to such parameters as thermal expansion coefficient, stiffness and creep resistance.
It is also well known in the prior art to render fillers of the foregoing type of increased compatability with the polymer matrix to improve the interfacial adhesion of the mineral to the matrix. Thus, for example, in Papalos, U.S. Pat. No. 3,227,675, kaolin clays are described, the surfaces of which are modified with organofunctional silanes. The kaolin clays so modified are used as fillers for natural and synthetic rubbers and the like. Additional references of this type include Iannicelli, U.S. Pat. Nos. 3,290,165 and 3,567,680. Similarly, in U.S. Pat. No. 4,789,403, a method is disclosed for producing a layered lattice silicate which is surface modified with an organic material. The layered lattice silicate is contacted with an organic monomer, comonomers, or a pre-polymer, and surface polymerization or reaction in situ is effected in the presence of a gaseous hydrogen atmosphere. Among the organic monomers that can be used in the process are various precursors of nylon.
More recently, processes have been disclosed which are said to be useful in producing composite materials composed of a polymer and a smectite-type clay mineral, with the mineral being connected to the polymer through ionic bonding. For example, in Kawasumi et al., U.S. Pat. No. 4,810,734 a process is disclosed wherein a smectite-type clay mineral is contacted with a swelling agent in the presence of a dispersion medium thereby forming a complex. The complex containing the dispersion medium is mixed with a monomer, and the monomer is then polymerized. The patent states that the swelling agent acts to expand the interlayer distance of the clay mineral, thereby permitting the clay mineral to take monomers into the interlayer space. The swelling agent is a compound having a onium ion and a functional ion capable of reacting and bonding with a polymer compound. Among the polymers utilizable are polyamide resins, vinyl polymers, thermosetting resins, polyester resins, polyamide resins and the like. Related disclosures are found in U.S. Pat. Nos. 4,739,007 and 4,889,885.
The swelling agents used in the Karasumi et al. and related patents cited above, technically qualify as organoclays. In the present invention as well, organically modified smectite-type clays, hereinafter referred to as "organophilic" or "organoclays", are used as the mineral component of the composite. In general, organoclays represent the reaction product of a smectite-type clay with a higher alkyl containing ammonium compound (often a quaternary), and have long been known for use in gelling of organic liquids such as lubricating oils, linseed oil, toluene and the like and for use as theological additives in a variety of organic based liquid systems and solvents. The general procedures and chemical reactions pursuant to which these organoclays are prepared are well known. Thus under appropriate conditions the organic compound which contains a cation will react by ion exchange with clays which contain a negative layer lattice and exchangeable cations to form the organoclay products. If the organic cation contains at least one alkyl group containing at least ten carbon atoms then the resultant organoclays will have the property of swelling in certain organic liquids. Among the prior art patents which discuss at length aspects of the preparation and properties of organoclays are U.S. Pat. Nos. 2,53 1,427, 2,966,506, 3,974,125, 3,537,994, and 4,081,496.
As utilized in the present specification, the term "smectite" or "smectite-type clays" refers to the general class of clay minerals with expanding crystal lattices, with the exception of vermiculite. This includes the dioctahedral smectites which consist of montmorillonite, beidellite, and nontronite, and to the trioctahedral smectites, which includes saponite, hectorite, and sauconite. Also encompassed are smectite-clays prepared synthetically, e.g. by hydrothermal processes as disclosed in U.S. Pat. Nos. 3,252,757; 3,586,468; 3,666,407; 3,671,190; 3,844,978; 3,844,979; 3,852,405; and 3,855,147.
The phase dispersions exhibited by the composite materials thus far discussed are relatively coarse, and differ materially in this respect from nanocomposites. The latter are a relatively new class of materials which exhibit ultrafine phase dimensions, typically in the range 1-100 nm. Experimental work on these materials has generally shown that virtually all types and classes of nanocomposites lead to new and improved properties when compared to their micro- and macrocomposite counterparts.
While the number of nanocomposites based on smectite-type clays and linear thermoplastics is growing, little work has been devoted to crosslinked polymeric systems such as epoxies. Recent reports of particulate-based epoxy composites suggest that the dimensional stability, conductivity, mechanical, thermal and other properties may be modified due to the incorporation of filler particles within the epoxy matrix. For the most part, however, the improvements in properties observed with these conventionally prepared composites are modest when compared (on an equal volume basis of particulate filler) to those that have been established for various polymer-ceramic nanocomposites.
Previous work by the present inventors on poly(imide), and poly(.epsilon.-caprolactone) have demonstrated the feasibility of dispersing molecular silicate layers within a macromolecular matrix, which results in significant improvements in physical properties with only modest particulate contents (&lt;10% by volume).
Wang and Pinnavaia have recently reported delamination of an organically modified smectite in an epoxy resin by heating an onium ion exchanged form of montmorillonite with epoxy resin to temperatures of 200.degree.-300.degree. C. Chemistry of Materials, vol. 6, pages 468-474 (April, 1994). X-ray and electron microscopy studies of the composite suggested delamination of the silicate layers, although phase segregation of the polyether-coated smectite from the epoxy matrix was observed. Furthermore, the product of the high temperature curing reaction is an intractable powder rather than a continuous solid epoxy matrix.
In accordance with the foregoing, it may be regarded as an object of the present invention to provide a smectite-epoxy nanocomposite which can be mixed, applied in various forms (e.g. as adhesive films, coatings, or castings), and cured by conventional means;
A further object of the invention is is to synthesize a polymer-ceramic nanocomposite in which smectite-type organoclays individual layers with a thickness of 10 .ANG. and a high aspect ratio (100-1000) are dispersed within a crosslinked epoxy matrix.
A yet further object of the invention, is to provide a process for the preparation of a smectite-epoxy nanocomposite which fulfills the above requirements, and is processed using conventional epoxy curing agents at temperatures significantly lower than those previously utilized.
A still further object of the invention, is to provide a process for preparing a smectite-epoxy composite, in which the resulting composite exhibits molecular dispersion of the silicate layers in the epoxy matrix, good optical clarity, and significantly improved dynamic mechanical properties compared to the unmodified epoxy.